The present invention relates to a force-reduction mechanism with a plurality of levers for a force-measuring device. The levers follow each other in a functional sequence where the force to be reduced is transmitted from one lever to the next by way of coupling elements. The force is introduced by way of a first coupling element into a first lever that is rotatably supported by a fulcrum on a stationary support. Mechanisms of the kind that the invention relates to have at least one additional lever beyond the first lever. The additional lever is connected by a coupling element to a preceding lever within the functional sequence of levers. Furthermore, in the kinds of mechanisms envisaged by the invention, the stationary support, coupling elements and levers are configured at least in part as portions of a monolithic material block.
Force-reduction mechanisms for a force-measuring device are known from EP 0 518 202 B1, where a force is reduced by means of at least one lever that is rotatably supported on a stationary support, the force being introduced into the lever by way of a coupling element. The stationary support, coupling elements and levers are configured as separate portions of a monolithic material block. In order to achieve large reduction ratios, embodiments are described which have two or three serially arranged levers connected by coupling elements. The levers are designed for a degree of structural strength commensurate with the load that each of them is exposed to, and they are essentially disposed at vertical positions immediately above one another. The fulcrum or pivotal axis of each lever, i.e., the resting point of the lever, is designed as a slender, flexible portion through which each lever is connected to the stationary support. If the input force into the mechanism is large, the fulcrum reactions of the levers will be of a corresponding magnitude. Consequently, the stationary support will have to be designed for adequate rigidity in the areas between its mounting portion and all of the fulcra. Thus, to meet a given minimum of structural rigidity, the stationary support needs to have the appropriate geometric dimensions. Accordingly, a certain predetermined portion of the monolithic material block has to be set aside for the stationary support. The customary dimensions of material blocks used in this application allow for two or possibly three levers and a stationary support of appropriate dimensions, together with a parallelogram linkage surrounding the force-reduction mechanism with two parallel-guiding members extending from a fixed leg to a movable leg of the parallelogram.
As mentioned in EP 0 518 202 B1, if a larger number of levers are provided in order to achieve larger reduction ratios, it will be necessary to either use a larger material block or to reduce the dimensions of the stationary support to allow for the additional lever volume. Each additional lever has to be supported by an adequately rigid portion of the stationary support. The levers following each other in sequence are arranged essentially parallel to each other, meaning that an imaginary longitudinal line defined by the fulcrum and the coupling pivots of each lever runs essentially parallel with the corresponding imaginary lines of the other levers. This imposes design limitations on the possible spatial arrangements for the levers, the stationary support, the fulcra and the coupling elements. The fulcra of sequentially adjacent levers are in alternating positions near opposite ends of their respective levers. Accordingly, the middle portion of the stationary support between the fulcra at opposite sides is weakened because of the material taken up by the levers. Thus, with a material block of customary size, it is impossible to increase the number of levers without a loss of structural rigidity. However, the use of a larger material block is undesirable, because the force-transmitting mechanism needs to be designed as a component fitting into an overall force-measuring system. If this one component were redimensioned, other components including standardized parts shared with other systems would likewise have to be changed.
It is therefore the object of the present invention, to provide a force-reduction mechanism in which large reduction ratios can be achieved in a compact space.
In accordance with the present invention, the foregoing objective can be met by a force-reduction mechanism with a plurality of levers following each other in a functional sequence. The force to be reduced is passed from one lever to the next by way of coupling elements. The force is introduced through of a first coupling element into a first lever that is rotatably supported by a fulcrum on a stationary support. Beyond the first lever, the mechanism according to the invention has at least one additional lever. The additional lever is connected by an additional coupling element to a lever that precedes the additional lever in the functional sequence of levers. The stationary support, coupling elements and levers are configured at least in part as distinct portions of a monolithic material block. In the force-reduction mechanism proposed by the invention, at least one of the additional levers has a fulcrum axis that is movable in relation to the stationary support.
The present invention is based on the observation that the use of only spatially fixed fulcra or pivotal supports for the levers represents a severe limitation on the possible layouts for additional levers and the possible reduction ratios. Given that the coupling elements connecting the levers can transmit tensile forces only, the connection from the second to the third lever at the latest will require a space-consuming reach-around portion to be added to one of the levers. For example in FIGS. 5 and 7 of EP 0 518 202 B1, already the first lever reaches laterally around the second lever. A reach-around portion of this kind needs to have a lever portion designed for a compressive load and a coupling element designed for a tensile load. The compressive lever portion reaches laterally from one lever around the other and thereby unnecessarily reduces the possible length of the lever that lies inside the reach-around portion. This represents an undesirable design limitation, given that in any event the levers have to be progressively shorter in order to allow each of the levers to rest on a fixed fulcrum on the stationary support.
It has been found that the force-reduction potential, i.e., the attainable reduction ratio in relation to the volume or to the largest side surface of the material block, can be increased by using at least one additional lever with a fulcrum axis that is movable in relation to the stationary support. In an advantageous embodiment of the invention, the movable fulcrum of a lever is located on a preceding lever. As is obvious in this embodiment as well as in general, a lever fulcrum that is fixed on a preceding lever participates in the movement of the preceding lever.
A preferred embodiment of the invention has a first, second and third lever arranged in a functional sequence. The respective fulcra of the first and second lever are located on the stationary support, while the fulcrum of the third lever is located on the first lever and thus shares the movement of the first lever.
A further developed embodiment of the inventive concept has four levers in a functional sequence. The respective fulcra of the first and second lever are located on the stationary support. The fulcrum of the third lever is located on the first lever and thus shares the movement of the first lever, while the fulcrum of the fourth lever is located on the second lever and thus shares the movement of the second lever.
The advantage of a movable fulcrum is that is requires no space on the stationary support. Also, the concept of a movable fulcrum provides more freedom in the design of force-reduction mechanisms. In particular, it eliminates the need for reach-around portions of levers, so that the freed-up space can be used for force-reduction elements. The levers with movable fulcra or pivotal axes allow a more space-efficient arrangement with respect to the attainable force-reduction ratio. Due to the advantage that no reach-around elements are needed for transmitting the force, the entire space taken up by each lever can be used to the benefit of the force-reducing function.
A lever that has its fulcrum axis fixed on a preceding lever of the force-reducing mechanism can be entirely surrounded by preceding levers. In an arrangement of this kind, the surrounded lever does not impose any design limitations on the shape of the stationary support nor on the way in which the dimensions and proportions of the surrounding levers are used to perform their force-reducing function. As in the known state-of-the-art devices, only a first part of the last lever in the force-reduction chain belongs to the monolithic material block. The remaining part of the last lever is an extension, attached to the first part by means of two bore holes. The extension is shaped like a fork, extending along both sides of the material block and ending, e.g., at a measuring or compensating device. The same two-part configuration is also possible when the first part of the last lever is surrounded by preceding levers. Even with more than two levers, a nested, spiral-like arrangement of a lever chain makes it possible to connect the successive levers only through coupling elements, i.e., without the need for reach-around, compressively loaded lever portions.
As can be demonstrated through model calculations and experiments, if at least one lever has its fulcrum axis on a preceding lever, the total reduction- or magnification ratio is slightly larger than the product of the ratios of the individual levers. The reduction ratio was determined for a mechanism with three levers where the fulcrum of the third lever was located on the first lever. In the example that was analyzed, the first lever had a short arm of 11.8 mm and a long arm of 66.2 mm. The second lever had arm lengths of 4.5 mm and 52 mm, respectively. The third lever had a short arm of 4.5 mm and a long arm of 101 mm, including a fork-shaped extension beyond the material block. The multiplication of the individual reduction ratios would lead to an overall ratio of 1/1455. With a correction allowing for the fact that the third lever fulcrum is located on the first lever, the effective reduction ratio turns out to be 1/1494. With the fulcrum of the third lever being located on the first lever, the correction leading to the effective reduction ratio depends on the distance between the respective fulcra of the first and third lever. In the subject case, the distance was 14.2 mm. This leads to the conclusion that the reduction ratio is increased if the fulcrum of a lever is located on a preceding lever of the mechanism. In addition to the space savings, the increase of the lever-reduction ratio is a further advantage of the inventive concept, according to which at least one lever rests on a fulcrum that is fixed on a preceding lever.
Also included among levers with a movable fulcrum are knee-joint levers. Knee-joint levers are force-reducing or or -magnifying devices with a force-introduction arm, a force-output arm, and an anchored arm, the three arms being connected to each other at a pivotal hub. When a force is applied to the force-introduction arm, the pivotal hub will be subjected to a slight displacement. Thus, by extension of the term fulcrum, the pivotal hub could also be called the movable fulcrum of the knee-joint lever.
In a knee-joint lever with a force-introduction arm, a force-output arm, and an anchored arm, the forces in the arms are in essence tensile forces, which can be transmitted through members with compact material cross-sections. The force-reduction ratio of a knee-joint lever is determined by an angle rather than by an arm-length ratio. Thus, very large reduction ratios can be achieved within the most compact space. Because of the essentially horizontal orientation of the first lever receiving the force to be reduced as well as of the last lever extending with its fork-like second part beyond the material block into a measuring or compensating device, it is necessary for coupling elements from the intermediate levers to the first and last lever to be vertical. Because a knee-joint lever changes the direction of the force between the input and output, the preferred arrangement has at least a pair of knee-joint levers with equal and opposite direction-changing angles. As a result, the change in force direction associated with the pair of knee-joint levers is essentially negligible.
To obtain an arrangement with only vertical or horizontal couplings, one would use knee-joint levers in which the force-introduction arm and the force-output arm are essentially at a 90xc2x0 angle to each other, while the angle between the force-output arm and the anchoring arm is 90xc2x0+"PHgr". The force-reduction factor of a knee-joint lever of this kind has the numerical value of tan("PHgr").
Because a knee-joint lever takes up only a small amount of space, the resulting loss in rigidity of the stationary support is insignificant. With the small amount of space required and with the possibility of prescribing the direction-changing angle, it is possible to design network-like arrangements of knee-joint levers with a very high degree of force reduction in relation to the space consumed. The knee-joint levers can be used either by themselves as intermediate levers, or also in combination with at least one additional straight lever. In the latter case, the additional straight lever is pivoted in a fulcrum located either on the stationary support or on another lever.
When there are more than two straight levers arranged in parallel and vertically above one another, at least one lever arm will have to transfer a force that is directed. towards a corresponding arm of the next lever. Because the coupling elements can only pull, but not push, i.e., they can carry only tensile forces, it is customary to design reach-around arrangements with a lever portion conducting a compressive (pushing) force and a coupling element transferring a tensile force. Instead of using a space-consuming arrangement of this kind, which contributes nothing to the force-reduction ratio, a combination of two consecutive knee-joint levers allows the pushing force to be converted to a parallel pulling force with a strong lever reduction taking place at the same time. Each of the two knee-joint levers redirects the force by an angle of 90xc2x0, so that the total change in direction is 180xc2x0. The change of 180xc2x0 in the direction of the force makes it possible to connect lever arms through a pair of knee-joint levers where otherwise a reach-around arrangement would have to be used.
It is self-evident that instead of the horizontally oriented straight levers it is also possible to use levers running in other, arbitrarily selectable directions. Knee-joint levers with appropriate direction-changing angles can be used to connect straight levers of different angular orientation. The use of knee-joint levers and/or straight levers that are fulcrum-supported on another lever significantly increases the design freedom to arrange levers in a material block. The increased design freedom allows large force-reduction ratios to be accomplished within a small amount of space.